An organic electroluminescence device (hereinafter the term “electroluminescence” is often abbreviated as “EL”) is a self-emission device utilizing the principle that a fluorescent compound emits light by the recombination energy of holes injected from an anode and electrons injected from a cathode when an electric field is impressed.
Since C. W. Tang et al. of Eastman Kodak Co. reported a low-voltage driven organic EL device in the form of a stacked type device (Non-patent Document 1, or the like), studies on organic EL devices wherein organic materials are used as the constituent materials has actively been conducted.
Tang et al. use tris(8-hydroxyquinolinol aluminum) for the emitting layer and a triphenyldiamine derivative for the hole-transporting layer. The advantages of the stack structure are to increase injection efficiency of holes to the emitting layer, to increase generation efficiency of excitons generated by recombination by blocking electrons injected in the cathode, to confine the generated excitons in the emitting layer, and so on.
Like this example, as the structure of the organic EL device, a two-layered type of a hole-transporting (injecting) layer and an electron-transporting emitting layer, and a three-layered type of a hole-transporting (injecting) layer, an emitting layer and an electron-transporting (injecting) layer are widely known. In such stack structure devices, their device structures and fabrication methods have been contrived to increase recombination efficiency of injected holes and electrons.
Heretofore, as a hole-injecting material used for an organic EL device, materials having a phenylenediamine structure disclosed in Patent Documents 1 and 2 have been known and widely used. Also, arylamine-based materials containing a benzidine skeleton disclosed in Patent Documents 3 and 4 have been used for a hole-transporting material.
On the other hand, Patent Documents 5 to 7 disclose arylamine-based compounds having a carbazole. These materials have the feature that when using these materials as the hole-transporting material, luminous efficiency is improved. However, there are drawbacks that driving voltage becomes significantly high at the same time, and that lifetime of the device becomes extremely short.
Patent Document 8 discloses a device using two or more hole-injecting layers in which the ionization potential values are stepwise set, in order to efficiently inject holes from an anode to an emitting layer. However, use of the material system disclosed in Patent Document 8 was insufficient in both the luminous efficiency and the life time.
[Patent Document 1] JP-A-H08-291115
[Patent Document 2] JP-A-2000-309566
[Patent Document 3] U.S. Pat. No. 5,061,569
[Patent Document 4] JP-A-2001-273978
[Patent Document 5] U.S. Pat. No. 6,242,115
[Patent Document 6] JP-A-2000-302756
[Patent Document 7] JP-A-H11-144873
[Patent Document 8] JP-A-H06-314594
[Non-patent Document 1] C. W. Tang, S. A. Vanslyke, Applied Physics, Letters, 51, 913 (1987)
An object of the invention is to provide an organic EL device having high efficiency and long life time.
An object of the invention is to provide a white light-emitting organic EL device having high efficiency and long life time.